The present invention relates to steam turbines and, more particularly, to steam turbines of the type having at least a high-pressure turbine and a reheat turbine wherein a reheat portion of an associated boiler adds heat to exhaust steam from the high-pressure turbine before the reheated steam is applied to the reheat turbine.
A medium or large steam turbine represents such a major investment to its owners that great care in its operation is essential to ensure completion of its operational lifetime. Major causes for concern exist during startup and loading of a steam turbine either from a cold or a hot condition. Specifically, the life of a steam turbine is critically affected by the thermal and mechanical stresses to which it is exposed during startup and loading.
As hot steam is admitted to a cold steam turbine, thermal gradients are produced between the outer and inner portions of the turbine rotor. A correlation can be made between the magnitude of the thermal gradients and a reduction in the number of times the steam turbine may be started from a cold or hot condition without overstressing the rotor and casing materials. For example, if the temperature of the incoming steam exceeds the external surface temperature of the turbine wheels by 400 degrees F. during each startup cycle, a typical steam turbine is capable of withstanding only one-fifth as many startup cycles compared to a steam turbine in which the steam-to-metal temperature difference is limited to about 300 degrees F. It can thus be seen that a difference of only 100 degrees F. in steam-to-metal temperature has a major effect on the lifetime of a steam turbine subjected to repeated starting and stopping.
In a steam turbine containing both a high-pressure turbine and a reheat turbine, coordinated control of thermal gradients is required in both turbines. Such coordinated control is complicated by the differences between the two turbines and the manner in which steam flows thereto.
In addition to the control of the thermal gradients, close control of turbine acceleration is also required during startup to ensure that inertially derived radial wheel stresses, added to thermally derived stresses, remain within tolerable limits until the rotors in each turbine stage become heat soaked.
The acceleration limits are especially severe in a steam turbine driving an electric generator prior to synchronization of the electric generator with the network line frequency because the speed of the steam turbine is acutely responsive to small variations in steam flow in this condition. After the generator is synchronized to the line frequency, the turbine speed remains essentially constant under control of the line frequency. Large capacity steam control valves normally employed during a turbine loaded condition may be somewhat imprecise to control steam turbine speed under low flow conditions usually encountered prior to synchronization.
The prior art employs a cold starting technique wherein the high-pressure and reheat steam turbines are warmed by injecting steam into the exhaust of a high-pressure turbine stages while the boiler temperature and pressure are brought up to operating conditions. A ventilator valve in the inlet piping of the high-pressure steam turbine is opened to exhaust part of the steam which has flowed in the reverse direction from exhaust to inlet.
The reheat steam turbine is mechanically and physically integrated with the high-pressure steam turbine on a common shaft within a common housing. Steam in the high-pressure steam turbine is normally sealed from flowing along the common shaft to the reheat steam turbine by an inter-turbine seal. While the steam is fed in reverse flow for warming the high-pressure steam turbine, the inter-turbine shaft seal is opened to permit a part of the warming steam to flow past the shaft seal into the reheat steam turbine for warming thereof.
The above warming technique using reverse steam flow through the high-pressure steam turbine depends on the accessibility of steam piping at the inlet end of the high-pressure steam turbine in which the ventilator valve may be installed. Certain types of steam turbines integrate the turbine inlet connections from the step valve piping the steam chests with multiple control valves and individual valve ports to the turbine inlet nozzle sections within a unitary casing. Such integration denies access to the inlet steam nozzle port passages for installation of a ventilator valve and makes warming by reverse steam flow less desirable.
Starting a hot steam turbine after a load rejection or a system trip, while boiler temperature and pressure are at, or near, full operating levels, also presents a problem of thermal gradients, particularly in the reheat turbine. Although the reheat turbine rotor is hot, its temperature is considerably below that of the steam available from the reheat portion of the boiler during the early stages of turbine loading. Thus, reduced turbine lifetimes, measured in the number of loading cycles which may be withstood, is possible.
The prior art also employs high-pressure and low-pressure bypass valves in an attempt to control the warming rate, turbine acceleration and speed while permitting sufficient steam to pass through the reheat portion of the boiler to prevent damage to the reheat portion boiler tubes.
The high-pressure bypass valve conventionally diverts superheated steam from the upstream side of the main bypass control valve to the cold side of the reheat portion of the boiler during the startup sequence. This bypass steam flow reduces the amount of steam which must flow through the high-pressure steam turbine and thus provides an additional control on the turbine speed.
The low-pressure bypass valve diverts steam from the hot side of the reheat portion of the boiler to the condenser instead of requiring it to flow through the reheat turbine. During the early stage of bringing the boiler up to operating conditions, the steam flow through the low-pressure bypass valve maintains a cooling flow of steam through the boiler tubes in the reheat portion of the boiler. During the early stages of turbine acceleration and loading, the low-pressure bypass valve aids in the control of the amount of steam fed to the reheat turbine and thereby aids in the control of turbine acceleration.
The above techniques using bypass valves are unable to provide satisfactory positive limits on coordinated heating of the two steam turbines, and of turbine speed during acceleration and the early stages of turbine loading during either a cold or hot start.